That’s exactly it mate. The terraces were just refaced and seats installed. There was no material change to the old earthworks that supported them. Whereas, the South Stand was completely remodelled.
I had no idea that Keppie had done some preliminary work on what a refurb might look like, but I would probably say that their solution, at first glance, looks like tinkering around the edges. If the South Stand could be dismantled and resited elsewhere, the rest of the stadium could be demolished allowing for a complete redesign. Whether or not dismantlement and salvage costs were to prove too costly though, is another story.
Presuming this all wasn’t cost prohibitive, the houses at the foot of Bolivar Street and the tenements at the end of Sommerville Drive would no longer be a hinderance to adding an additional tier(s) if the footprint was moved (predominantly) southwards.
The constraints of the existing relief of the land would be overcome and the issues that you rightly mention (were a multi-purpose, i.e. athletics sympathetic, design to be adopted) could be addressed.
Maybe Reichsfuhrer Sturgeon could be persuaded to chip in properly?